1. Field of the Invention
This invention relates to a laminated tube container used for packing therein toothpaste, food or the like. More particularly it relates to a laminated tube container comprising a silicon oxide deposited layer.
2. Description of the Related Art
Laminated tube containers used for packing therein toothpaste, food or the like which are conventionally in wide use are those in which a sheet material mainly comprised of a plastic film is used in the body. In particular, for the purpose of preventing the contents from their deterioration due to oxidation caused by oxygen coming from the outside of laminated tube containers, a material with excellent oxygen barrier properties, water-vapor barrier properties, etc. as exemplified by a metal foil such as aluminum foil and a plastic film with gas barrier properties such as polyvinylidene chloride film or ethylene-vinyl alcohol copolymer film is laminated together with a polyethylene layer, an ionomer resin layer or the like serving as a thermoadhesive resin layer (hereinafter often "sealing medium"). Of these materials, the aluminum foil is most widely used because of its very high gas barrier properties.
In the meantime, conventional laminated tube containers make use of aluminum foil as stated above and hence they are inevitably opaque. In recent years, however, there is an increasing demand for laminated tube containers which have high gas barrier properties and whose contents can be seen through. A transparent, laminated tube container that can answer such a demand is disclosed, for example, in Japanese Utility Model Application Laid-open No. 63-64638. The transparent, laminated tube container disclosed therein has the body comprised of a sheet material comprising a polyethylene terephthalate film and a deposited layer of a metal oxide such as silicon oxide, formed thereon as a gas barrier layer. Techniques by which such a deposited layer of silicon oxide or the like is formed as a gas barrier layer are known from relatively old times, and are reported in, for example, Japanese Patent Publications No. 51-48511 and No. 53-12953, and Japanese Utility Model Publications No. 52-3418 and No. 52-24608.
The silicon oxide deposited layer as disclosed in the Japanese Patent Publication No. 51-48511, Japanese Utility Model Publication No. 52-24608, etc., however, is so extremely thin a glassy layer that fine cracks tend to occur, which seriously tend to occur especially when heated. This is presumed to result from the phenomenon that the substrate on which the silicon oxide deposited layer is formed undergoes expansion and contraction by heat and the silicon oxide deposited layer can not follow this expansion and contraction. Occurrence of such cracks results in a serious lowering of the gas barrier properties of the laminated tube container.
An occasion on which such a thin silicon oxide deposited layer is heated can be met when a thermoadhesive resin layer is formed on the silicon oxide deposited layer in order for the laminated tube container to be endowed with thermal adhesion properties. As a function required in laminated tube containers, laminated tube containers must have the thermal adhesion properties so that the laminated tube containers can be readily worked when they are formed into tubes. In order for the laminated tube containers to be endowed with the thermal adhesion properties, the substrate for a laminated tube is laminated with a thermoadhesive resin film (a sealing medium) in usual instances. In such instances, the sealing medium usually gives a layer having the largest thickness in the sheet material for the laminated tube, and also gives a layer that imparts strength and flexibility to the laminated-tube sheet material and serves as a main component layer of the laminated-tube sheet material. A method for its lamination may include a method in which the substrate is laminated with a thermoadhesive resin film previously prepared, and a method in which a thermoadhesive resin is directly extruded in the form of a film onto the substrate to effect lamination to form the sealing medium, i.e., what is called the extrusion coating. In the method in which the substrate is laminated with the sealing medium, a substrate on which the silicon oxide deposited layer has been formed and the thermoadhesive resin film (sealing medium) previously prepared may be laminated using an adhesive, or the substrate and the sealing medium are laminated interposing between them a thermoadhesive resin coating formed by extrusion. The latter enables easy formation of the sealing medium in a large thickness, and is a preferred method.
Almost all of the laminated tubes making use of metal foil which are commercially available at present are comprised of a sealing medium formed by extrusion coating of the thermoadhesive resin onto the metal foil.
However, as previously stated, in the direct extrusion coating of the thermoadhesive resin onto the silicon oxide deposited layer, the substrate may undergo expansion and contraction in the manner as described above, to cause occurrence of fine cracks in the silicon oxide deposited layer. In addition, the occurrence of cracks may be promoted because the thermoadhesive resin layer formed by extrusion coating undergo constriction as the layer is cooled. As a result, the gas barrier properties of the laminated tube containers may be greatly lowered. This has been a problem unsolved.